1. Field of the Invention
The present invention relates to minimizing the effects of parasitic drive voltage on a rate sensor and, more particularly, to a simple and efficient device and method for accurately sensing motor motion and rotation rates.
2. Description of the Related Art
Micromachined silicon rate sensors are expected to provide tremendous cost and size advantages over many competing technologies. Micromachined rate sensors are typically single crystal silicon "resonating structure" gyros that are rugged, inherently balanced, physically small, and inexpensive to fabricate. Using batch processing techniques that are typical in today's semiconductor industry, thousands of identical sensors, each less than a few square millimeters in area, can be fabricated simultaneously. The utilization of micromachining process technology provides a major advantage of size and cost over existing macroscopic devices. Small micromechanical gyro chips capitalize on the inherent low costs associated with silicon wafer fabrication techniques while providing sensitivity and ruggedness suitable for many uses, such as in automobiles.
A rate sensing gyro (RSG) is analogous to an undamped spring-mass mechanical system. In broad terms, the principle of operation uses a resonating structure device that, under initial excitation, will induce a velocity in the structure sensing masses, e.g., proof masses. The proof masses will oscillate at the natural resonance frequency (NRF) of the spring-mass system when excited. This oscillation can be maintained by an external amplification circuit which feeds back energy at the correct frequency and phase to replace any losses attributable to the mechanical structure. When an angular rate is applied to the oscillating system, comprised of the proof masses coupled to springs, a corresponding force is induced on the masses which is very linearly proportional to the magnitude of the applied angular rate. This force can then be applied to produce an electrical signal which may be calibrated and which serves as a rate indicator.
Electrostatic combs may be utilized to drive the masses with oscillatory signals. Small changes in the motion of the vibrating masses occur when the device is rotated about a sense axis. The sense axis is parallel to the plane in which the masses move, and normal to their velocities. The Coriolis effect causes the masses to oscillate slightly out of their original plane of motion, by an amount proportional to the rotation rate. Coriolis forces have been described as forces that act orthogonally (at 90 degrees) to the motion of the masses. Accordingly, measurement of the mass deflection allows the determination of the rotational rate.
Low cost rate sensors, however, have not been without certain disadvantages. It has been found that one drawback in micromachining technology is that it is often difficult to extract accurate voltage signals representing the sensed rates. To measure such small signals, complicated and extremely sensitive electronics are typically necessary. However, undesirable parasitic feedthrough of the drive voltage signal into the processing electronics tends to affect the output sense signal, distorting the signal by the amount of the drive signal. This parasitic drive voltage exists due to imperfections in the sensor manufacturing and electronics operations and process, whereby a capacitive coupling mechanism couples the drive voltage directly to the motor position output signal and the sensed rate signal.
The two coupling arrangements act as error sources. Drive voltage is orthogonal to motor position, and drive voltage coupling to motor position causes a rotation in phase with the motor position signal. However determination of the correct phase of the motor position is critical to signal processing of the sensed rate. In addition, drive voltage is in phase with sensed rate, such that coupling of the drive voltage into the rate sense pickoff induces a false rate "bias." This false bias adds an error term which is very large compared to angular rates that would ordinarily be sensed. Drive voltage-induced bias is typically a factor of 100 greater than the desirable full range of actual sensed rate.
For example, the sensor sense axis produces a charge signal that is proportional to the sensed rate. Typically, the rate sensor output magnitude of charge is on the order of 6.times.10.sup.-18 coulombs per degree per second of sensed rate. This signal is very small and, when converted to voltage, becomes indistinguishable from noise for small sensed rates. Thus, because the sensed signal tends to be adversely affected by the drive voltage pickup when the drive voltage is at the NRF of the masses, it becomes critical to limit parasitic capacitance or voltage which has been shown to be a source of noise amplification.
The problem of drive voltage feedthrough into the sensed rate has been addressed by frequency multiplexing the sensed rate and applied motion at the NRF. Frequency multiplexing has been accomplished by applying a separate frequency tone to the proof mass sense plates. Applied angular rate multiplies this tone to produce a sensed rate tone which is a product of the NRF applied tone and the applied rate. In addition, signal processing requires that the sensed signal be demodulated at the sensed tone and the NRF. This method effectively eliminates drive voltage feedthrough into the sense pickoff. However, the problem of undesirable coupling of the drive voltage into the sensed motor position is not eliminated.
More particularly, as shown in FIG. 1, a micromachine comb drive circuit 100 operates as a conventional oscillator. Upon initial power on, there is no oscillation. A certain amount of noise, however, is generated in the circuit, and is amplified around the loop comprised of amplifiers 112, 116, and 124. This noise component is attributable to flat white noise in which there is a component that is in phase with the NRF of the sensor which, in turn, causes that component of the signal to continue to grow without bound. The signal ultimately becomes bounded by the action of the multiplier 118, the low pass filter 120, and the automatic gain control element 122.
The signal amplified by amplifier 116 is detected by an RMS converter including multiplier 118 and low pass filter 120, which converts the signal to a DC voltage. The DC value is applied to one terminal of the automatic gain control device 122. Consequently, the signal originating from the amplifier 116 is fed forward to a compensator 110 and to another amplifier 124, and is input into the second terminal of the automatic gain control 122. The automatic gain control element 122 regulates the amplitude of oscillation to precisely control the amplitude of motion. The compensator performs the function of converting position to velocity to maintain the phase around the loop at zero to sustain oscillation.
Furthermore, another attempt to eliminate feedthrough includes incorporating multiple frequencies into the motor by utilizing the "square law" nature of conversion of the drive voltage into motor force. That is, drive voltage applied to the outer combs of the motor produces a force on the motor which is the square of applied voltage (V.sup.2). When the applied force is the NRF plus a DC preload voltage component, the force becomes (DC+NRF).sup.2. This expands to DC.sup.2 +2*DC*NRF+NRF.sup.2. It is recognized that the middle term "2*DC*NRF" produces force in the motor. One method for avoiding feedthrough at the NRF requires that the motor be driven with two additive frequency tones, neither of which is the NRF. However, when the tones are squared in the motor forcing transfer function, a force component which is at the NRF is produced. Thus, any drive voltage frequency tone at the NRF of the masses is eliminated.
As illustrated in FIG. 2, it can be seen that the operation of the multiple frequency oscillator is similar to the fundamental oscillator with the addition of a reference frequency 232. The second frequency may be an arbitrary value multiplied (at 224) with the NRF of the sensor and summed with the product at the summing amplifier 228. Accordingly, the signal is amplified, and the resultant signal is the product of the arbitrarily selected reference signal and the motor NRF summed with the reference frequency. The resultant signal is applied to the amplifier and, consequently, to the motor. The output of amplifier 230 is then amplified and regulated in the automatic gain control circuit 222, amplified at amplifier 212, and then applied to the sensor 214. The sensor 214 converts voltage to force which squares the applied force such that the resultant frequency is at the motor NRF.
In contrast with the device of FIG. 1, the two-frequency oscillator differs in the amplification and regulation loop with the addition of the multiplier 224, the summing amplifier 228, and the reference frequency 232. However, it has been found that the system efficiency is only one-fourth of that of the fundamental motor frequency oscillator. The two-frequency oscillator tends to generate undesirable intermodulation tones that are difficult to keep out of the signal processing electronics. These spurious tones are an undesirable by product of the two-frequency drive. The tones are degrading if they are coincident with a sensor "out of sense plane" mode. These sensor modes are unavoidable and rejection of sensors for modes coincident with spurious tones are typically expensive to screen, and tend to limit the product yield.
In addition, spurious tones can occur in the sensed rate output whenever they lie within the processing passband. This can occur by direct injection or can occur because they are aliased into the passband as a result of sensor rate processing. Filtering for unwanted tones, however, places an added burden on the sense processing circuitry. Thus, the drive circuitry tends to be more complicated and expensive than the fundamental motor drive oscillator. However, although the two-frequency oscillator may be less desirable in certain cases than the fundamental motor frequency oscillator, there may be instances where it is advantageous to use the two-frequency oscillator to conquer the application limiting problem of feedthrough.
Thus, it can be seen that although the methods described above solve some of the problems of parasitic drive voltage pickup, these methods accomplish this at the expense of complex modulating and demodulating circuitry or by spurious tone-producing summation methods that require summing amplifiers and multiplier a to extract the desired signals. Both of these methods require a considerable amount of circuitry which is difficult to efficiently implement in a low cost integrated circuit process.